The present invention is generally directed to a novel and improved process for the recovery of metal, salt and oxide values present in secondary or low grade aluminum dross. More particularly, this invention is directed to a process for recovering the metallic aluminum, metal salt, and aluminum oxide components present in secondary aluminum dross. In this regard, an important aspect of the present invention specifically concerns a closed loop process for treating secondary aluminum dross to recover metallic aluminum, eutectic mixtures of sodium and potassium chlorides suitable for re-use as salt fluxes in aluminum smelting furnaces, and an aluminum oxide product having high aluminum fine and low chloride contents suitable for use as an exothermic topping in the production of steel ingots.
During the smelting of aluminum scrap, such as used beverage containers and the like, large quantities of salt flux, typically sodium and potassium chloride are added to the smelter furnaces. This flux accumulates on top of the molten bath and forms a dross which, for the most part, has been either disposed of in its entirety in landfills, or subjected to limited processing wherein only a portion of the aluminum values contained therein are recovered with the remainder being transported from the processing site for landfill disposal.
A currently utilized procedure for the processing of secondary aluminum dross or salt cake involves initially reducing the particulate size of the dross, typically by impact and ball milling processing. Following particle reduction, the dross is then melted under a salt flux cover to dissolve the contaminants therein, mostly aluminum oxide, and recover the aluminum metal. Typically, rich dross (containing 30 to 70% aluminum) and low-grade dross (containing 5 to 25% aluminum) are melted in rotary furnaces, using a sodium chloride-potassium chloride flux which may include 2 to 5% cryolite. As the flux becomes contaminated with aluminum oxide, it is removed as salt slag by skimming or raking and solidified. Salt slag generated by this process generally ranges up to 35% of the melt charge in the furnaces. While this salt slag contains potentially valuable materials that could be recovered, it is a general practice to pay for its removal from the production site and disposal in a landfill. Furthermore, soluble salts present therein represent a potential source of pollution to surface and ground water supplies. Accordingly, it is becoming increasingly important both for economic as well as environmental reasons, to eliminate the need for landfill disposal of these salt slags.
Other previously employed commercial procedures for processing secondary aluminum dross have generally involved crushing the aluminum dross chunks into smaller particles and then, without size separation, leaching those particles on a continuous basis.
In general, all of these prior commercial procedures do not permit maximum recovery of aluminum values and also produce aluminum oxide and other residues which also require landfill disposal.